1. Field of the Invention
There are two basic ways to achieve higher microwave power levels: device combining and circuit combining. Device combining is normally achieved by paralleling plural device units, thereby creating in effect a larger device with higher power output. This invention takes the circuit combining approach wherein a flow of microwave power is divided among the inputs to plural amplifier circuits whose outputs are combined to yield a flow of greater power. The invention is an improvement in the part of a device that divides or combines power, known as a power divider/combiner.
2. Description of the Prior Art
Power amplification with broad bandwidth at microwave frequencies was the exclusive domain of traveling wave tubes until the relatively recent advent of solid-state devices that can provide reasonable gain and power output above 6 GHz. The development of relatively efficient solid-state power combining schemes has made possible medium-power solid-state sources with performance comparable to or exceeding that of traveling wave tubes. These solid-state sources have incorporated developments in the field of field-effect transistors (FETs), resulting in improved efficiency and linearity along with greater reliability and lower size, weight and system cost. Furthermore, power supply requirements have been simplified because transistors require low voltages as compared to traveling-wave-tube high voltage supplies, with their accompanying insulation and reliability problems.
A recent approach in the development of modular FET power divider-amplifier-combiner systems, taught in U.S. Pat. No. 4,234,854, has been the use of radial microstrip line divider/combiners. In this approach, power is divided and fed to a first set of radial microstrip transmission lines, with each line feeding a separate elemental amplifier unit. The output of each amplifier unit feeds one of a second set of radial microstrip transmission lines. The second set of radial transmission lines conducts power to a point where the power is combined and conducted from the system.
Radial power divider-amplifier-combiner systems of the type described above present difficult impedance matching problems. The usable bandwidth of such systems that use FETs as amplifiers is principally determined by the control of the load impedance with frequency as seen by the output of each elemental FET amplifier unit. This load impedance directly affects the gain and saturated output power of the unit. Therefore, an effective system should provide a voltage standing wave ratio (VSWR) of 1.25:1 or less to the output of the FET amplifier units over the full operating bandwidth. One approach to impedance matching, taught in U.S. Pat. No. 3,582,813, has been to use transmission lines that taper in width from the outer circumference of the set of radial lines to the center dividing/combining point. The impedance transforming in the radial microstrip transmission lines requires high characteristic impedance sections to achieve the broadest bandwidth performance. However, the wide tapered microstrip transmission lines have a low characteristic impedance, which sacrifices potential bandwidth. High characteristic impedance lines (greater than or equal to 80 ohms) are extremely narrow width lines that are impractical to define, and also have large dissipative loss when compared with low characteristic impedance transmission lines of 50 ohms or less. A sufficiently high characteristic impedance and low power loss required in many applications cannot be realized simultaneously using conventional divider-amplifier-combiner designs.